1. Field of the Invention
The present invention relates to a process for preventing a formation of dust during the loading and shipping of sulfate fertilizer granulates.
2. The Prior Art
Fertilizers are manufactured to an increasing extent in the granulated form and shipped in loose bulk. In order to produce the granulates, dry pressing granulation is used extensively. This shapes the salt particles into shells, from which granulates are subsequently produced by crushing the shells. After screening, the granulates are preferably in the grain size range of 1.0 to 4.0 mm.
It is known to add auxiliary granulating agents to the starting mixture of fine-grained substances which are granulated to produce granulates. Thus, for example, German Patent No. 1,242,249 discloses a process for the production of granulated mixed fertilizers from calcium cyanamide and potash fertilizer with solid urea or urea nitrate as an auxiliary agent in roll presses, whereby about 3% solid urea is used with the other starting materials. In addition to the pressing process, build-up granulation (also roll granulation) is widely used as well.
For example, according to German Patent 1,012,932, lime mixed with a nitrogen-containing fertilizer is dry mixed with urea and is then granulated with 2% to 10% water, and subsequently dried. The granulates produced by pressing have sharp corners and edges caused mainly by the subsequent crushing process, which are rubbed off during the shipping or transloading of the material. Then, during the shipping and during the further use of the granulates, this results in an undesirable formation of dust, causing an unacceptable polluting of the environment.
It is known that attempts have been made by the producer to counteract this pollution phenomenon by screening finest-grain prior to shipping. However, this type of after-treatment alone does not suffice to remove the adhering fine residual dust or to enhance the resistance to abrasion of the edges, corners and also sides of the granulate grain.
A process is known from German No. 136,956 for improving the resistance to abrasion of potash fertilizer granulates by subjecting such fertilizer to an after-treatment after the granulating process. This after-treatment includes dust removal in a fluidized bed, followed by a subsequent treatment of the granulate surface with water or aqueous additives, with downstream drying and cooling of the granulate so treated. The process is based on the possibility of eliminating still-present unstable points or edges by slight solubilizing and recrystallization, and of solidifying the granulate surface in this way.
German OS 3,003,183 discloses that at a temperature range of 80.degree. C. to 100.degree. C., the treatment method achieves an optimal gain in strength if a minimum residence time of 10 seconds under an atmosphere of high humidity is simultaneously used. The specified treatment method requires a high energy expenditure for the actual granulation and screening in the downstream plant and equipment. This process is only feasible by the producer, because the heat required for such process originates from the granulate production.
Consequently, this prior art method is not applicable for granulated potash products that have to be stored for a long period of time in intermediate storage facilities and then loaded again from there, in addition to the investment expenditure required and the maintenance cost involved. Further, disadvantages are due to the longer storage time and the physical process variables connected therewith, such as storage pressure and change in the atmospheric humidity. These can have negative effects on the treated granulates, such as a tendency to cake together and a softening of the grain.
Additional use has been made of dust-bonding agents, for example, mostly organic substances, which are added to the granulate in small quantities, so that a protective cover is produced and dusting is strongly reduced. A mixture of soft paraffin and spindle oil has been used for this purpose in German 2,538,276.
In German OS 3,918,523, a solution of molasses and another oxygen-containing hydrocarbon such as glycerin, polyethylene glycol, and triethanolamine is sprayed onto a fertilizer granulate, in particular onto potassium sulfate, for bonding the dust.
However, other types of substances have been proposed. For the most part, these are foreign substances without fertilizing properties. Furthermore, they are now increasingly deemed ecologically undesirable because they may lead to, for example, annoying odor or, in the case of fertilizer, pollution of the ground water supply.